Modern pressures on natural resources have decreased their availability and in the case of fish has made this important protein source more difficult and expensive to obtain in the quantities required by our increasing population. As a result, the ancient art of fish farming or fish culture is flourishing and numerous ponds across the country are being constructed or devoted to raising fish as a crop.
The productivity or yield of a fish pond is a direct function of the dissolved oxygen contained in the water. The fish farming industry attempts to maintain pond populations at a maximum and this results in the average fish farmer operating his ponds at the brink of disaster. As the fish increase in size, their oxygen demand increases and the amount of ammonia and nitrate excreted by the fish increases which increases other plant life in the pond. This plant life creates oxygen when exposed to sufficient quantities of solar radiation but at night it utilizes the dissolved oxygen in the pond. Thus the oxygen level can suddenly diminish to the critical point and the fish begin to die. The first indication that a farmer has of a potential oxygen depletion problem is the fish gasping for oxygen at the surface of the water in the middle of the night. When this stage of oxygen depletion has been reached, it is generally too late and the fish are belly up by morning and the investment in the oxygen depleted pond is lost with the rotting fish suitable only for fertilizer.
Oxygen depletion has been combated in the past by a number of systems but they are all costly to operate and result in decreased profits from the average fish pond. In many instances their operation becomes so costly that fish farming ceases to be a profitable enterprise. One approach to the oxygen depletion problem is the concept of running aerators all night long to ensure that critical oxygen levels are not exceeded. This constant running of aerator systems prevents oxygen depletion but, as previously stated, becomes so costly that profits are minimized and in some cases financial losses occur.
The aerating systems utilized vary from very crude mechanical devices which flail the water surface and are powered by tractors to the very efficient spray type aerators such as those manufactured by Rodale Resources Incorporated which sprays pond water into the air in the form of water droplets dimensioned to minimize evaporation but maximize oxygen absorption. None of these devices regulate the amount of oxygen in the water and therefore their operation must be manually controlled and the process of maintaining adequate amounts of dissolved oxygen in the pond becomes inefficient as previously discussed.
Systems which automatically maintain a predetermined oxygen content in a water source have been developed but they are more costly to operate than continuously running the prior art mechanical aerators. For instance the device disclosed in the O. C. Smith U.S. Pat. No. 3,320,928 on "Apparatus And Method For Aerating A Body Of Water" teaches the concept of placing a large dome over the surface of the water and forcing oxygen gas through a pipe having an opening near the bottom of the body of water so that the oxygen bubbles up through the water. The oxygen trapped under the dome and the air contained therein are recirculated with the oxygen pumped to the bottom of the pond in an attempt to increase the efficiency of the system. However, this system does not increase the oxygen level of the pond as readily as does the mechanical spray system and oxygen gas is costly relative to the market value of the fish in the pond. The mechanism for recirculating the oxygen is also an added expense which decreases the overall efficiency of the fish farming system utilizing such devices.
J. Walker, U.S. Pat. No. 3,872,003 on "High Oxygen Treatment Of Waste With Selective Oxygen Recirculation" and E. Abrams et al, U.S. Pat. No. 4,069,147 on "Waste Water Treatment With Oxygen" are further examples of the concept of circulating oxygen gas through a body of liquid in response to signals from an oxygen sensor located within the liquid. These systems are not directed to fish culture but the techniques are applicable to the industry. They suffer from the same disadvantages discussed with respect to the O. C. Smith method of aerating a body of water by pumping oxygen gas therethrough.